Remote speaker systems with integral amplifiers are sometimes equipped with automatic turn-on circuits. These controls automatically energize the power amplifier upon receipt of a low level audio signal and allow the power amplifier to shut down after a given period of time after the cessation of the input signal.
Typical applications include automotive booster amplifiers for stereo systems, remote speakers for stereo systems, or public address installations. In some of these uses, it is simply prudent to reduce the quiescent current drain of the amplifier when no power amplification is required. In other cases, battery operation of the amplified speaker requires that power be conserved by disconnecting the main amplifier from the power source whenever no input signal is present.
One type of automotive automatic turn-on and turn-off system is disclosed in U.S. Pat. No. 4,453,264 issued June 5, 1984 to the inventor of the subject invention. In the automotive environment, the stereo signal comprising left and right input channels are treated identically. The input signal is AC coupled to a differential amplifier stage which generates a ground referenced signal. The signal is AC coupled to a voltage comparator. The comparator in each channel is set to develop a positive going output pulse whenever the input exceeds a predetermined level, generally 30 mV. The output pulses which follow the input signal excursions are or'ed by diodes and feed an integrator capacitor. The voltage across the integrator capacitor is monitored by a MOSFET switch which in turn pulls in a power control relay. Typical transistor characteristics allow the device to turn on whenever the voltage across the integrator capacitor reaches a predetermined level, approximately 4 V. In the absence of any audio input signal, the charge on the capacitor dissipates slowly through a bleeder resistor so as to shut off the FET and drop out the relay after some delay. A problem with this system is that the turn-on sensitivity is linked directly to turn off time delay. Longer delays reduce the turn-on sensitivity because relatively large capacity required in the integrator, and the subsequent delay in reaching the 4 volt turn-on point. An additional problem with type of technology is that human hearing being essentially logarithmic in sensitivity can easily detect very low audio levels in the existing speakers if they are used. The auxiliary, amplified speakers should switch on automatically at this low level, and not at higher levels where their sudden turn-on could be very annoying. Random line noise may invariably trigger on the system. Reducing the turn-on sensitivity by increasing the comparator set point does render the system more noise immune, but increases the turn-on delay.
A second system is disclosed in an article titled "Sound Operated Switch" written by Michael Tooley and David Whitfield in Practical Electronics, May 1979 edition. The article discloses a switching circuit which turns on and off in response to an audio frequency signal. The incoming audio frequency signal is applied to an amplifier stage and then to a precision unity gain active full wave rectifier. The output excursion of the rectifier is applied to the comparator whose output is used to enable a conventional timer circuit, which is a 555 timer integrated circuit. It stresses that once the input level has risen sufficiently to trigger the circuit and enable the timer, the output of the monostable will remain high regardless of any subsequent reduction of input level. The problem with this type of design is that the system is not retriggerable. The integrated timer circuit is not retriggerable due to the fact that it receives an input signal and produces an output signal for the predetermined time after which it goes low and waits for another input signal. This would cause blanking out of the speaker.